The present invention relates generally to implantable medical devices and more particularly to implantable medical devices intended for use in monitoring a patient""s heart rhythm and abnormal respiration.
Implantable pacemakers and cardioverters monitor the heart""s rhythm in order to detect arrhythmias and deliver appropriate therapies to terminate detected arrhythmias. In conjunction with this function, the ability of the device is to store information with regard to monitored heart rhythms has dramatically increased over the past two years. Examples of implantable pacemakers and defibrillators which have the capability of storing information related to monitored heart rhythms include U.S. Pat. No. 5,330,513 issued to Nichols et al., U.S. Pat. No. 6,129,745 issued to Sun et al. and U.S. Pat. No. 5,447,519 issued to Peterson. In addition, there have recently been developed subcutaneously implantable monitoring devices that do not deliver any anti-arrhythmia therapies to the heart but simply store information regarding a patient""s heart rhythms for later uplink to an external device. Such devices are disclosed in U.S. Pat. No. 5,851,221 issued to Rieder et al., U.S. Pat. No. 5,535,752 and U.S. Pat. No. 5,564,434 issued to Halperin et al.
In conjunction with implantable devices as described above, information stored relating to a patient""s heart rhythm may include information relating to heart rate trends over time, as disclosed in U.S. Pat. No. 5,088,488 issued to Markowitz et al., U.S. Pat. No. 4,364,397 and U.S. Pat. No. 4,360,030 issued to Citron et al., as well as information relating to heart rate variability over time, as disclosed in U.S. Pat. No. 5,957,861 issued to Combs et al., U.S. Pat. No. 6,045,513 issued to Stone et al. and U.S. Pat. No. 5,876,353 issued to Riff.
Typically, measurements of heart rate trend in such devices are accomplished by continually measuring heart rate over a defined time period, and calculating average heart rates for successive shorter time periods within the defined time period for later telemetry to an external device. Gradual increases in average heart rate over extended time periods are known to be an indicator of decompensation, a phenomenon that takes place during the progression of clinical heart failure.
The present invention is directed toward an implantable device having enhanced capabilities for monitoring a patient""s heart rate and respiration trends over extended periods of time. The information collected by the implantable device is stored and telemetered to an associated external device such as a device programmer for display and analysis. Heart rates are measured by measuring the time intervals between sensed depolarizations of a chamber of the patient""s heart and preceding sensed depolarizations or delivered pacing pulses. Intervals may be measured in the ventricle and/or atrium of the patient""s heart. The measured intervals are referred to hereafter as xe2x80x9cheart intervalsxe2x80x9d. The measured heart intervals during defined time periods are used to calculate average heart rates or average heart intervals associated with the time periods. Preferably the average heart rate takes the form of a mean heart rate, but in some embodiments, the median heart rate over the time periods may be employed or the most common heart rate or interval based on a histogram of measured heart intervals or other equivalent value may be substituted. For purposes of the present application, the term xe2x80x9caverage heart ratexe2x80x9d should be understood to include mean, median or any other equivalent values indicative of the general heart rate or heart interval.
Rather than simply measuring average heart rate values over successive time periods, the implantable device instead measures successive average values of heart rates measured during discontinuous time periods, preferably chosen to occur during times of particular interest, for example during defined time periods during the night and/or day. Preferably the measurements are taken and stored over a period of weeks or months. In a first embodiment, measurements are during the night during a period of time in which the patient is likely to be sleeping. In this context, measurement of the trend of night heart rates taken, for example over the period of time between 12:00 a.m. and 4:00 a.m. is believed to be particularly valuable. Night heart rate is predominantly controlled by the parasympathetic nervous system. The progression of heart failure is usually associated with abnormal excitation of the parasympathetic nervous system, leading to increases in night heart rate.
In addition, long-term trends of daytime heart rates may also be collected, for example over periods of time between 8:00 a.m. and 8:00 p.m. Daytime heart rate is primarily controlled by the sympathetic nervous system and thus differences in day and night heart rates can be used as a measure of autonomic dysfunction and have been shown to be different in heart failure patients when compared to age matched individuals with normal hearts. In the context of an implantable pacemaker, comparisons of trends of day and night heart rates to the lower or base pacing rate of the pacemaker may also provide useful physiological information. This comparison may be especially valuable in pacemakers which store information regarding trends of physiologic sensor outputs or regarding trends of pacing rates based upon physiologic sensor outputs as in U.S. Pat. No. 6,045,573 filed May 13, 1998 by Stone et al, incorporated herein by reference in its entirety.
In a preferred embodiment of the invention, the implantable device includes a sensor indicative of exercise level either measured directly using a physiologic sensor such as an accelerometer or piezo-electric sensor or measured indirectly by means of a sensor of metabolic demand such as a pressure sensor, oxygen saturation sensor, stroke volume sensor or respiration sensor. In this embodiment of the invention, measurements of heart rhythms are made only in response to the sensor""s determination that the patient is at rest, in order to produce a long-term trends of resting heart rates during the defined time intervals. Even over relatively long time frames, a patient""s level of activity may vary substantially, and changes in average heart rates can be masked by such variations in exercise level. By limiting the measurements of heart rates to times during which the patient is known to be at rest, a more accurate indication of the true long-term progression of heart rates can be obtained. In such embodiments the implantable device may collect heart rate information continuously during longer time periods, typically extending at least over several hours. During the longer time periods the device may define a series of shorter time periods, typically extending over several minutes, and will employ heart rate information collected during a preceding one of the shorter time periods only if the sensor indicates the patient was at rest during the shorter time period.
In some preferred embodiments, particularly those intended for use in patients known to suffer from tachyarrhythmias, the implantable device is also configured to reject intervals between depolarizations associated with tachyarrhythmias. In such embodiments the implantable device may define a minimum cumulative duration of non-rejected heart intervals as a prerequisite to calculation of an average rate value for a defined time period.
In devices employing physiologic sensors, the device may correspondingly also store values indicative of the general levels of sensor output during daytime and nighttime periods may also be collected. In such embodiments, average sensor output values, including the various types of averages discussed above in conjunction with calculation of average heart rates may be employed. Alternatively, a sum or total of all generated sensor outputs during relevant time periods may be employed.
According to one aspect of the invention, the physiologic sensor is an implanted impedance sensor employed to measure respiration rates. Systems using impedance sensors to measure patient respiratory trends are disclosed in U.S. Pat. No. 5,957,861 issued Combs et al, U.S. Pat. No. 5,876,353 issued to Riff et al, or U.S. Pat. No. 5,562,711 issued to Yerich et al, all incorporated herein by reference in their entireties.
As discussed in the cited patents, respiration rates are often tracked using minute ventilation. Minute ventilation is defined as the total amount of gas that is moved into, and out of, the lungs in one minute. This measurement is generally obtained in a clinical stetting using a flow meter positioned within a patient""s mouth. However, the inventors have shown that minute ventilation can also be closely approximated by measuring the changes in tissue impedance that occur as the lungs expand and contract during breathing, as may be detected by an implanted impedance sensor.
The current invention provides a system and method for monitoring minute ventilation in a manner that extends beyond the clinical setting so that long-term trends in patient health may be more accurately evaluated. For example, the invention may be used to detect otherwise unrecognized acute disease or acute deterioration in the status of chronic disease. Early detection of otherwise unrecognized acute disease permits treatment that can potentially prevent, or minimize, further progression of the disease.
According to one aspect of the invention, minute ventilation is recorded when the patient is at rest as determined by an activity sensor or a metabolic rate sensor. In another embodiment, minute ventilation is recorded at predetermined time periods such as between the hours of twelve midnight and two in the morning. Short and long-term changes in minute volume can be used to detect conditions such as Cheyne-Stokes respiration and sleep apnea.